1. Field of the Invention
This invention relates to aiding vascular surgery, including improving nature and use of prosthetic grafts, particularly for by-passes to relatively small arteries.
2. Description of Related Art
By-passes required to save limbs can be long even stretching from a groin to below a knee. The arteries to which by-passes are connected may be as small as one to five millimeter across. Where patients have no suitable veins to use as by-passes, which is often the case for patients with relevant serious conditions, the only positive alternative is use of prosthetic grafts made of synthetic materials, for such as flexible tube of PTFE (polytetrafluoroethylene). Justification would become questionable without reasonable success rates and/or improvement prospects. Making simple direct end connections or anastomoses of prosthetic graft tubes (usually run at an acute angle or more or less parallel with the artery and end cut at an angle) to side apertures in arteries, perhaps particulary arteries substantially less than five millimeters across, has been followed by such liability to fibrous intimal hyperplasia as later to lead to serious blood flow reduction, even stoppage. Such problems have led to various theoretical considerations and assumptions, including as backing to more complex practical proposals.
We are particularly familiar with one such practical proposal, namely use of a small piece of natural vein to make a short cuff that is joined by surgical stitching to and between the artery opening and the end of the prosthetic graft tube. Improved success rates for indirect prosthesis-to-vein-to-artery connection, compared with direct prosthesis-to-artery, have involved reduced adverse effect from intimal hyperplasia. Contributory factors, for cuff-type and other prosthesis types, have been considered and postulated as including reducing tendencies to turbulence of blood flow, and/or optimising approximation to laminar blood flow, and/or for suppleness of the natural vein parts to aid absorption or cushioning blood pulsing. These factors have further been seen particulary as contributing to avoiding or minimising occurrence of artery wall shear stress.
This invention has arisen from questioning such theorising and assumption, particulary after reflecting on the fact that success of autologous natural vein cuffs has included using cuff sizes so large as to make us doubt that either of laminar flow or pulse absorption would be likely to occur. Close observation of careful simulations has not only supported that view, but has further led us to develop successful proposals based on contradicting both of the prior objectives for maximising laminar flow and minimising artery wall shear stress.